And the first thing that came to mind for me was how DID they shed heat?

Maybe that's what those super-necks and -tails were for; to increase
the surface area:volume ratio to help cool down. Feeding adaptations
and defense may well have been secondary.

What of the 'spikes' found on some diplodocid necks/tails? Could they
have had rich internal blood supplies to further the cooling process
(something similar has been suggested for stegosaur plates)? The frill
on Amargasaurus also comes to mind.